Existing electric storage devices having a high energy density represented by, for example, electric double layer capacitors and lithium-ion secondary batteries are produced by the following method.
Sheet-shaped electrodes are each produced by applying an active material such as active carbon, a lithium-containing double oxide, or carbon to a current-collecting foil constituted by a metal foil such as an aluminum foil or a copper foil.
Such two sheet-shaped electrodes are subsequently arranged such that the active-material layers thereof face each other and a sheet-shaped separator is interposed between the electrodes to prevent a short circuit caused by direct contact between the electrodes. Subsequently, the electrodes and the separator are wound or units each constituted by the electrodes and the separator are laminated to provide a laminated body in which pairs of the positive electrode and the negative electrode are laminated.
To these positive and negative electrodes, aluminum tabs or nickel tabs are welded as outer electrodes by ultrasonic welding or the like to provide electrode groups.
These electrode groups are placed in a package constituted by an aluminum can, an aluminum laminate film, or the like. The package is filled with an electrolytic solution and then sealed. Thus, an existing electric storage device is obtained.
However, use of such an existing method is difficult to achieve further reduction in the size or profile of electric storage devices.
Thus, Patent Document 1 discloses, as an electric storage device that allows further reduction in the size and profile, an electric double layer capacitor described below in detail.
Patent Document 1 discloses the electric double layer capacitor (electric storage device) as follows. Active carbon electrode layers are formed on surfaces of aluminum collector electrodes and the collector electrodes are arranged such that the active carbon electrode layers thereof face each other. The peripheries of these collector electrodes are provided with heat-bonding parts that are formed of, for example, modified polypropylene or modified polyethylene and that are melted by heating. A separator is disposed and an electrolytic solution is supplied between the collector electrodes. The heat-bonding parts are then heated to bond (thermocompression bond) the collector electrodes together and to seal the collector electrodes so as to contain the electrolytic solution therein. Thus, a unit cell (electric storage unit) is formed. Such unit cells are optionally laminated. Thus, the electric double layer capacitor is provided.
The heat-bonding parts have a function of maintaining the shape of a unit cell and preventing a short circuit caused by contact between collector electrodes. Thus, the heat-bonding parts allow further reduction in the size and profile of electric storage devices.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-313679
In the electric double layer capacitor (electric storage device) in Patent Document 1, the collector electrodes are sealed by completely covering the peripheries thereof. Accordingly, after the unit cell is formed by melting the heat-bonding parts, an electrolytic solution cannot be supplied into the unit cell (or laminated body).
For this reason, an electrolytic solution needs to be injected between collector electrodes prior to heating of the heat-bonding parts, that is, prior to formation of the unit cell. In this case, heat applied during thermocompression bonding may cause alteration or evaporation of the electrolytic solution.
In addition, after injection of the electrolytic solution and prior to thermocompression bonding, the electrolytic solution may leak during lamination of electrodes (collector electrodes) and adhere to equipment or the like, which causes difficulties in handling.
In the obtained electric storage device, the electrolytic solution is contained between the sealed collector electrodes. Accordingly, gas generated by decomposition of water or impurities in the electrolytic solution and the like is not discharged from the laminated body. Thus, the generated gas may cause, for example, swelling of the laminated body (in between electrodes), an increase in the impedance (internal resistance), a decrease in the capacitance, or separation between component members such as separation between a collector electrode and an active-material layer.